Persons constructing and operating facilities for handling volatile liquids and petroleum products and derivatives thereof are confronted with difficult and challenging problems as respects the control of portions of these products in vapor phase. Such vapors are fugitive and commonly escape to the atmosphere in quantities presenting economic losses and series hazards to the health of humans, animals and plant life. Private and commercial interests have made earnest and costly efforts to minimize these vapor emissions and hazards but the complexities of the problems to be dealt with and the costs involved present obstacles which have not been satisfactorily resolved heretofore. In recent years governmental agencies have endeavored to establish strnginet standards butressed with stiff penalties. Despite these incentives and the diligent efforts of many designers in providing a wide variety of modes of dealing with the problems no satisfactory protective system has yet been devisedmeeting current requirements.
The following patents are typical of the vapor treating and recovery system heretofore proposed, namely, Hartman U.S. Pat. No. 2,765,872, Moragne, U.S. Pat. No. 3,266,262, Bragg U.S. Pat. No. 3,590,559, Battey U.S. Pat. No. 3,714,790, Husa, U.S. Pat. No. 3,783,911, Schonewald U.S. Pat. No. 3,648,436, David, U.S. Pat. No. 3,672,180, Henrix U.S. Pat. No. 3,830,040, McNamee U.S. Pat. No. 3,886,759, and Mair U.S. Pat. No. 3,903,078. These systems are either totally lacking or have inadequate means for preventing the occurrence of flammable mixtures in the recovery system and, in consequence, are subject to hazardous operating conditions. Only Husa U.S. Pat. No. 3,783,911 and McNamee U.S. Pat. No. 3,886,759 utilize oxygen analyzers to monitor vapor for the presence of oxygen but each of these teachings lacks suitable means for utilizing the analyzer to provide protection against flammable vapor conditions. Some prior vapor recovery systems are said to operate with a reasonable degree of satisfaction under limited ambient temperature conditions as, for example, a temperature range of 70.degree.-80.degree. F, but provide unacceptable and unsatisfactory results under commonly encountered higher ambient temperatures, such as 85.degree.-120.degree. F.
Among the vapor control expedients currently in use are those utilizing floating roofs with seals between the roof rims and the interior tank walls. These systems are costly, difficult to maintain, subject to varying vapor emissions under varying structural and atmospheric conditions, which losses are extremely difficult to measure with any degree of accuracy. They also fail to meet the continually more stringent standards promulgated by governmental environment agencies. A predominant cause of vapor leaks from floating roof seals is the out-of-roundness of the tank shell. Such distortions develop over a period of time with earth movements under the tank. Ambient weather conditions also tend to deteriorate the effectiveness of the working floating roof seal.
Closed vapor recovery systems, on the other hand, using a blanketing gas under fixed tank roofs have no working seals, and 100% of liquid vapor emissions are normally recovered. Prior to this invention such systems have been hazardous for lack of any reliable provision to safeguard against explosive conditions owing to the possible entry of oxygen into the system under various operating conditions. Ignition of such combustible gas mixtures can occur from numerous sources, such as electrostatic conditions caused by fluid movements within the storage tanks, tank gauging operations, or the presence of pyrophoric iron sulfide, and others. Also, liquid contaminations have occurred between tankages in a closed system, and constant operator attention has been necessary. Commercial vapor recovery packages commonly are designed for truck loading rack vapors exclusively and use higher compression ratios, sometimes to auto-ignition pressures, refrigeration with attendant icing problems, and/or absorption with up to 10% of the collected vapors being wasted in order to dispose of that which does not absorb.